This invention relates to squelch circuits for communications systems and more particularly to squelch circuits used in multiple carrier communications systems.
In some communications systems, specifically those utilizing amplitude modulation, multiple carrier techniques are employed to provide extended range communications. One example of a communications system using multiple carrier techniques is the ARINC system which handles the exchange of operational information between an airplane flight crew and a ground station, normally in an airline office. The ARINC system is comprised of a network of mainly unattended ground based transceivers which transmit and receive VHF or line-of-sight carriers. The transceivers are arranged in linearly dispersed groups, all transceivers being effective to transmit or receive simultaneously. A message originating on the ground for an airborne aircraft is transmitted by the group of transceivers closest to the known position of the aircraft, a known calling technique being employed to excite only the receiver of the called aircraft. As might be suspected the called aircraft will usually hear simultaneous transmissions of the identical message from multiple transceivers. The constraints of the ARINC system require that each transceiver in a group transmit on a different frequency than other transceivers in the same group but these frequencies are closely spaced to one another. Thus, considering the closely spaced multiple carriers, a number of heterodyne frequencies are produced in the airborne receiver. More specifically, in the present ARINC system, in the carrier range of 118 MHz to 138 MHz, carrier offsets of 2.5, 4.0, 5.0, 7.5 and 8.0 kHz are encountered in practice. These produce difference frequencies in the airborne receiver AM detectors of 4.0, 5.0, 7.5, 8.0, 10.0, 12.0, 12.5, 15.0 and 16.0 kHz depending on the particular carrier environment in which the aircraft is operating, that is, the number of carriers that are impressed on a receiving antenna at one allocated channel frequency and their relative frequency offsets.
The difference frequencies produced in this multiple carrier environment render a conventional signal to noise squelch ineffective since this type of squelch cannot distinguish difference frequencies from normal Gaussian noise. For example, the conventional signal to noise squelch is comprised generally of the serial arrangement of a noise filter, noise detector, low pass filter and a comparator which controls a squelch gate. In the absence of an incoming carrier the noise filter input from the receiver AM detector is predominately noise. This noise is applied through the noise filter to the noise detector which in response thereto generates a noise signal which is applied through the low pass filter to the comparator. Here the noise signal is compared against a DC reference to close the squelch gate whenever the noise signal exceeds the reference. When a valid carrier and information signal are received, the receiver AGC action reduces the noise level from the receiver AM detector, thus reducing the noise signal to open the squelch gate. However, in a multiple carrier environment the resultant difference frequencies produced by a valid input signal generally fall within the band pass of the conventional squelch circuit noise filter and will produce a noise signal output from the noise detector to close the squelch gate. In that event, the airborne receiver will miss some communications or transmissions addressed to it.
Certain sophisticated squelch systems known in the prior art avoid the above mentioned problem, however, these are seldom used in practical receivers due to cost and complexity. Accordingly, the most common method of avoiding lost communications in current practice is a manual disabling of a conventional noise squelch on user command. This is obviously less than satisfactory since receiver noise will then be present as audio output. In addition, this manual scheme is undesirable as it requires a user operated control and necessary interface wiring to the receiver squelch circuits.
Other techniques rely on complex carrier recognition processes involving phase locked loops at intermediate (IF) or higher frequencies. These systems are complex, difficult to maintain and can be rendered ineffective in the presence of three or more carriers on an assigned channel.
Sharp narrow band noise filters which respond to valid information frequencies are impractical since normal transceiver frequency stability will insure that difference frequencies will be found essentially anywhere within a 4 to 16 kHz bandwidth.